The present invention relates to a Group III-V nitride semiconductor laser device, emitting laser light at a short wavelength, which is expected to be applicable to optical information processing, for example.
In recent years, apparatuses for various types of large-capacity optical disks, including a digital versatile disc (DVD) apparatus, have been put into practical use, and yet research and development have been carried on to further increase the storage capacity of an optical disk. As is well known in the art, shortening the oscillation wavelength of laser light for use in reading and writing information is one of the most effective ways to realize an apparatus adapted to read and write information from/onto an optical disk with an even larger capacity. A semiconductor laser device, which is built in a DVD apparatus currently available, is often made of a Group III-V semiconductor compound (typically, AlGaInP). The oscillation wavelength of an AlGaInP semiconductor laser device is 650 nm. Accordingly, in order to implement a semiconductor laser device compatible with a high-density DVD apparatus under development, the laser device should oscillate at an even shorter wavelength by using a Group III-V nitride semiconductor compound.
Hereinafter, a conventional Group III-V nitride semiconductor laser device will be described with reference to FIG. 10.
FIG. 10 illustrates a cross-sectional structure of a conventional Group III-V nitride semiconductor laser device.
Referring to FIG. 10, a GaN buffer layer 102 and an n-type GaN contact layer 103 with a low resistance are formed in this order on a sapphire substrate 101. On an active region of the n-type contact layer 103, an n-type AlGaN cladding layer 104, an n-type GaN optical guide layer 105, a multi-quantum well (MQW) active layer 106, a p-type GaN optical guide layer 107 and a p-type AlGaN cladding layer 108 are stacked in this order. In the MQW active layer 106, plural pairs of Ga.sub.1-x In.sub.x N well layers and Ga.sub.1-y In.sub.y N barrier layers (where 0&lt;y&lt;x&lt;1) are alternately stacked. The p-type cladding layer 108 includes a ridge stripe portion 108a with a width ranging from about 3 .mu.m to about 10 .mu.m on the upper surface thereof.
A p-type GaN contact layer 109 with a low resistance is formed on the upper surface of the ridge stripe portion 108a of the p-type cladding layer 108. And both side faces of the ridge stripe portion 108a, the upper surface of the cladding layer 108 except for that portion 108a, and the side faces of the multilayer structure formed on the active region are all covered with an insulating film 110.
A striped p-side electrode 111 is formed on the insulating film 110 to come into contact with the p-type contact layer 109. And an n-side electrode 112 is formed on the n-type contact layer 103 beside the active region thereof.
When the n-side electrode 112 is grounded and a predetermined voltage is applied to the p-side electrode 111, the semiconductor laser device with such a structure enters a laser oscillation state at a wavelength in the range from 370 nm to 430 nm. This oscillation wavelength is variable with the composition and thickness of Ga.sub.1-x In.sub.x N and Ga.sub.1-y In.sub.y N layers forming the MQW active layer 106. Continuous laser oscillation of such a laser device at room temperature or more was already accomplished. Thus, it is expected that such a laser device will be available everywhere in the near future.
However, in the present state of the art, the conventional nitride semiconductor laser device can maintain continuous laser oscillation at no shorter than about 370 nm. In view of the operating principle of the laser device, it is impossible to further shorten the oscillation wavelength thereof.
Generally speaking, to shorten the oscillation wavelength of a semiconductor laser diode, a so-called "wide-gap semiconductor" with a larger energy gap, which is also termed "band gap", may be used as a material for the active layer thereof. In the MQW active layer 106, for example, the In mole fraction x in Ga.sub.1-x In.sub.x N may be 0 (i.e., GaN may be used instead) or AlGaN, containing Al with a larger energy gap, may be used. In either case, a shorter wavelength is attained for the device.
In a semiconductor laser device with a double heterostructure adapted to confine carriers and produced light within the active layer thereof, the cladding layers thereof should be made of a semiconductor with an even larger energy gap than that of the active layer thereof.
In general, in order to actually operate such a laser device at room temperature or more, the cladding layers thereof should have an energy gap, which is larger than that of the active layer thereof by at least about 0.4 eV. Since the energy gap of AlGaN semiconductors is changeable within a broad range from 3.4 eV to 6.2 eV, cladding layers with a larger energy gap can be formed. However, if the composition of an AlGaN semiconductor is changed to have a larger energy gap, it is harder to turn the conductivity type of the semiconductor into p-type by doping the semiconductor with a p-type dopant. This is because a thermal activation efficiency of holes decreases in such a case. Accordingly, in the prior art, even if the AlGaN semiconductor is doped with a p-type dopant, the Al mole fraction in AlGaN is at most 0.2 (i.e., a mixed crystal Al.sub.0.2 Ga.sub.0.8 N) and the energy gap thereof is at most about 4.0 eV.
As described above, in the conventional Group III-V nitride semiconductor laser device, the energy gap of the p-type semiconductor layer thereof can be no greater than about 4.0 eV.
Moreover, when crystals with a relatively large Al mole fraction are stacked over crystals with a relatively small Al mole fraction, strain is caused between crystals of these two types, because the lattice constants thereof are different from each other. Accordingly, if semiconductor crystals with a large Al mole fraction are grown to form a cladding layer until the thickness thereof reaches a required value of 1 .mu.m or more, then the semiconductor crystals are likely to crack. As a result, the characteristics and the reliability of the laser device with such a cladding layer deteriorate.